1. Field of the Invention
The present invention relates to thermodynamic heat engines and heat cycles for the conversion of heat or thermal energy directly into work or more useful forms.
2. Description of Prior Art
As sensors and actuators become smaller, there is a continuing and growing need for micro-power sources that efficiently convert heat energy into more useful forms at very small physical scales. The technology to build extremely small heat engines is available, but there are currently no heat engines or heat cycles that work efficiently, if at all, at such small physical dimensions.
Fossil fuels are a non-renewable energy source that must be used wisely. Unfortunately, all known and currently available heat engines and thermodynamic heat cycles prevent the efficient conversion of energy from valuable non-renewable resources. That means we are consuming precious, non-renewable energy resources today in inefficient thermodynamic processes and whatever energy is wasted in those processes is lost forever.
Some ferroelectric materials, such as barium titanate (BT) and lead zirconate titanate (PZT), experience a drastic change in the dielectric constant, or relative permittivity, when heated to a temperature, called a Curie point, of the particular material. The dielectric constant increases in magnitude very rapidly just before the Curie temperature is reached, and it drops again sharply as the temperature increases further. There have been attempts in the past to use this characteristic of ferroelectric materials for the conversion of heat directly into electricity where the ferroelectric material is used as the dielectric in a parallel-plate capacitor. Hoh describes and claims in U.S. Pat. No. 3,243,687 dated Mar. 29, 1966 and entitled “Energy Converter” an invention exploiting what was defined as the “thermo-dielectric effect” which utilizes the rise or fall in the dielectric constant of dielectric matter in a capacitor as it is heated or cooled. Hoh anticipated using the increase or decrease in voltage and thus, the amount of energy gained in an energized capacitor with isolated electric charges when it is heated or cooled for the direct conversion of thermal energy into electricity. Ferroelectric direct-conversion devices of this nature are impractical because a capacitor that has a dielectric with temperature-dependent permittivity must be physically moved through a heat cycle. Some devices even require a cyclical heat source. Moreover, the conversion efficiency of such devices is very low because getting heat effectively into and out of the dielectric material of the capacitor has proven to be a difficult task.
3. Objects and Advantages
It would therefore be advantageous to provide a heat engine that does not require a cyclical heat source, has a minimum of moving parts, and converts thermal energy into kinetic energy or more useful forms very efficiently at even the smallest physical scales. Moreover, it would also be advantageous to provide a heat engine and heat cycle using the wide variation of the dielectric constant, or relative permittivity, of certain materials around a Curie temperature that results from heating or cooling the materials. A heat engine that first converts thermal energy efficiently into motion or kinetic energy would also be advantageous over the less efficient methods available for converting thermal energy directly into electricity.
A dielectrophoretic heat engine uses the extreme variation in the dielectric constant of a ferroelectric material, such as barium titanate (BT) or lead zirconate titanate (PZT), at temperatures near a Curie point, or a Curie temperature, of the material, but in a different and more efficient way to convert heat energy directly into mechanical energy. Simply put, a capacitor that has a dielectric with temperature-dependent permittivity is not physically moved through a heat cycle to produce electricity as was done previously, but the dielectric moves instead under the influence of an electric field in a different heat cycle, and the resultant kinetic energy of the moving dielectric can then be converted to electricity. Adequate explanation of the dielectrophoretic heat engine and associated heat cycle, which are the objects of the present invention, requires a definition and understanding of a new principle. The “thermodielectrophoretic effect” is hereby defined as the tendency of an electric field to attract and draw in matter with a temporary higher relative permittivity, while simultaneously rejecting or displacing other matter with a temporary lower relative permittivity from the same electric field, where the variance in relative permittivity between the two sections of matter is caused by a temporary temperature difference between the sections of matter. In other words, dielectric matter with a temperature-dependent permittivity is used as the working substance in a heat engine where the heat cycle consists of heating and cooling sections of the dielectric matter so that sections of dielectric matter with a temporary higher permittivity are drawn into an electric field while other sections of dielectric matter with a temporary lower permittivity are simultaneously displaced from the same electric field. The result of the drawing in or displacement of, sections of dielectric matter with a temperature-dependent permittivity results in a net movement or motion in one direction of the dielectric matter into and through the electric field.
Consider the example of a square parallel-plate capacitor in which a square dielectric plate is inserted part way into the gap between the electrode plates. Using the conventional approximate equations for the properties of a parallel-plate capacitor, it can readily be shown that the electrostatic field pulls the dielectric slab toward a central position in the gap with a force, F, given byF=V2(e1−e2)a/2d, where V is the potential applied between the electrode plates, e1 is the permittivity of the dielectric slab being drawn into the electric field, e2 is the permittivity of air that is being displaced from between the electrodes, a is the length of an electrode plate, and d is the thickness of the gap between the plates. Typically, the force is small from a macroscopic human perspective. However, the above equation shows that the force depends on the ratio between the capacitor dimensions but does not depend on the size. In other words, the force remains the same if the capacitor and the dielectric slab are shrunk to very small dimensions (nanometer size). At the same time, the masses of all components are proportional to the third power of their linear dimensions. Therefore, the force-to-mass ratio and, consequently, the acceleration that can be imparted to the dielectric slab are much larger at very small physical scales than at the macroscopic scale. The present invention exploits this effect, and thus, engines can be built that are more powerful, efficient, and effective than other heat engines at very small physical scales. Certain substances, such as barium titanate (BT) and lead zirconate titanate (PZT), exhibit a temperature-dependent permittivity near a Curie temperature, Tc. For that matter, the permittivity varies greatly within a span of a few degrees above or below Tc. Furthermore, the Curie temperatures of some substances can be permanently altered with the addition of certain quantities of other substances, such as lead or strontium. Usually, the addition of lead increases the Curie temperature, and the addition of strontium tends to decrease the Curie temperature. That means a variety of substances can be made by doping which exhibit a continuum of Curie temperatures ranging from extremely high temperatures down to room temperature or below. The present invention exploits the concept of temperature-dependent permittivity along with the ability to create substances where each substance exhibits a different Tc in order to increase conversion efficiency by cascading heat engine stages where each stage uses dielectric matter with a slightly different Curie temperature as the working substance.
4. Objects and Advantages
Accordingly, besides the objects and advantages of the heat engine and associated heat cycle as described above, several advantages of the present invention are:                (a) to provide heat engines capable of converting heat with relatively small variations in temperature directly into mechanical energy or motion;        (b) to provide heat engines that have multiple stages with working fluid material used in the engine having different Curie temperatures in order to attain very high Carnot efficiencies;        (c) to provide simple heat engines which have relatively few moving parts and which can be produced cost-effectively;        (d) to provide heat engines that can be constructed to operate at any physical scale;        (e) to provide relatively powerful heat engines which are cost-effective, lightweight, and small;        (f) to provide heat engines capable of converting low-grade heat near room temperatures directly into mechanical energy or motion;        (g) to provide means and methods of cooling electronic circuits and components with very small dimensions;        (h) to provide means and methods for pumping or moving matter very efficiently at small physical scales.        